Due to the advantages of non-toxicity, low smoke density and low moisture absorption, cyclotriphosphazene flame retardants are generally used for flame-retarding modification of materials, such as epoxy resins, unsaturated resins, polyurethanes, cyanates, benzoxazines and the like. In order to increase the flame retardant efficiency of cyclotriphosphazene flame retardants, cyclotriphosphazene-functional groups and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) or dioxacyclophosphate are generally integrated into the same flame retardant molecules to form synergistic flame retardants. Phosphorus elements in phosphaphenanthrene or phosphate play a role through gas-phase flame-retardant mechanism and mechanism of catalytic char formation together with cyclotriphosphazene intumescent flame retardant mechanism, so as to improve the flame retardant efficiency of the flame retardants, to reduce the usage amount of flame retardants and to give the materials excellent overall performance.
The document (Muraki, T., et al. Polymer Degradation and Stability. 2004, 84(1):87-93), CN104478934A and U.S. Pat. No. 4,405,738(A) all disclose that cyclotriphosphazene flame retardants containing dioxacyclophosphate have an obviously improved flame-retarding efficiency as compared to cyclotriphosphazene flame retardants. The aforesaid documents all show that the combinations of phosphates with cyclotriphosphazene flame retardants are advantageous to increasing the flame-retarding efficiency of flame retardants. However, phosphate groups having a bad thermal resistance are independently bonded to the end of the flame retardant, and water absorption is too great, which are not conducive to the application of such flame retardants in the fields of CCL which have higher performance requirements on heat, water absorption and the like.
CN101993456A and CN104262399A disclose gradually replacing phosphates with DOPO structures having better thermal resistance and lower water absorption for the preparation of flame retardants containing cyclotriphosphazene and phosphaphenanthrene at the same time. CN103435652A further discloses a process for preparing a novel compound having high nitrogen content and containing phosphaphenanthrene and phosphazene double functional groups. Although the flame-retarding efficiency of these flame retardants is obviously improved, high density accumulation of aromatic groups in flame retardant molecules results in an extremely low dissolving property of such flame retardants in common low-boiling solvents such as butanone, acetone, toluene, ethylene glycol methyl ether, propylene glycol monomethyl ether. Such flame retardants can only be dissolved by adding a larger volume of strong polar organic solvents, such as N,N-dimethylformamide, N,N-diethylacetamide and dimethylsulfoxide. These solvents having a high boiling point bring much trouble to the subsequent processing. High density accumulation of aromatic groups also renders undesirable compatibility of flame retardants in resin systems. As compared to common micromolecular cyclotriphosphazene flame retardants, it tends to agglomerate and precipitate in the resin after the solvent is removed, which not only is not conducive to improving the flame-retardant property of resins, but also leads to uneven structures of resins in the molding process, so as to affect the overall performance of the resins.
Meanwhile, the cyclotriphosphazene flame retardants containing a phosphaphenanthrene structure currently disclosed have a melting point of generally about 160 to 200° C. Therefore, when the flame-retardant modified products by using such flame retardants are subject to heat processing, such as drilling, soldering and the like, the flame retardant readily melts and precipitates, resulting in product defects such as dried flowers, hollow and the like, which are seriously threats to the product life.
CN102199294A discloses a hyperbranched polysiloxane and a process for preparing the same, wherein said hyperbranched polysiloxane contains both a phosphaphenanthrene structure and an organosilicon epoxy resin. Although it has a better compatibility in resin systems, its flame-retardant property needs to be further improved. Moreover, its hyperbranched structure is more complex.
CN102250147A discloses a process for preparing cyclotriphosphazene containing silicon functional group, and a use thereof. The cyclotriphosphazene containing silicon functional group contains both cyclotriphosphazene and siloxane structure. Although polypropylene obtained by applying such flame retardant into polypropylene systems has a higher elongation at break, its oxygen index is only about 25%, and the flame retardancy is still to be further improved.
Accordingly, it is desirable in the art to obtain a halogen-free flame retardant which is capable of both enhancing the flame-retardant property and improving its compatibility in resin systems.